1. Field of the Invention
The present invention relates to an electrophotographic developing apparatus, such as a copier, a printer, or a facsimile apparatus, which renders an electrostatic latent image visible by using a toner in an electrophotographic process, and to an electrophotographic apparatus using the same.
2. Description of the Related Art
In a developing apparatus used in an electrophotographic image forming apparatus as an electrophotographic apparatus, toner particles reduced in size improve denseness of an image and reproducibility of extremely small dots, thereby enabling development with high image quality. However, it has been known that the toner particles reduced in size increase a surface area per unit volume, which increases the amount of frictional charging per unit volume or per unit weight, i.e., the specific charge of the toner and lowers developability.
For example, Japanese Unexamined Patent Publication JP-A 5-216274 (1993) discloses a contact development method using a one-component non-magnetic toner having a first surface region and a second surface region such that charging of a specified polarity occurs at the first surface region, while no charging occurs at the second surface region. The use of such a toner prevents an excess increase in the amount of charge even when a particle diameter is reduced. In the prior art technology, however, the composition of the toner is complicated.
When the specific charge of a toner is reduced to increase the amount of development, i.e., the amount of the toner adhering to an electrostatic latent image, an image force is lowered so that the ability to cause the toner to adhere to a toner holding member is reduced. For this reason, there has conventionally been required to coat a toner with a maximized mechanical force or by applying a supply bias voltage between a supply member and the toner holding member and thereby promote the supply of the toner by using such coating means. When a toner having a small particle diameter is coated continuously under mechanical pressure, the toner is more likely to be fixed to the toner holding member, which presents a problem. Normally, the amount of toner used for development corresponds to about two toner layers so that it is sufficient to coat and transport toner in a minimum required amount. If the coating means is used by performing mechanical coating or applying a bias voltage as described above, an excess amount of toner is coated so that a large amount of toner should be removed by using a regulating member. Mere lowering of the specific charge for an increased amount of development is undesirable and disadvantageous in terms of the supply and transportability of the toner.
In supplying toner, if the toner is highly charged one, namely, has a large amount of charge, the needed but minimum amount of the toner can be coated under low stress without the coating means mentioned above by performing mechanical coating or supplying a bias voltage. On the other hand, the amount of the toner consumed in a development region is reduced and the amount of the toner supplied to an electrostatic latent image is reduced, so that the density of image is lowered and the amount of development is thereby reduced. Thus, ease of supply and the amount of development are contradictory to each other.